Lasers are now commercially available which produce light pulses having durations of about one picosecond (ps) or a little less (subpicosecond). They are obtained by synchronously pumping a dye laser using a continuous wave (c.w.) mode-locked pumping laser (argon or frequency doubled YAG).
However, there is no commercially available laser capable of delivering pulses that are shorter than 0.5 ps, even though pulses of a few tens of femtoseconds (fs) duration can be obtained in the laboratory. Such pulses are highly unstable in duration, amplitude, and spectral characteristics.
It should also be observed that the operation of femtosecond lasers is highly complex. Heretofore, they have been described only in qualitative terms. No satisfactory quantitative model has been proposed, given the very high number of parameters involved.
In Appl. Phys. Letters 38, 671 (1981), R. L. Fork, B. I. Green, and C. V. Shank describe a dye laser comprising an assembly including a saturable absorbent in a ring structure. The dye amplifier assembly was continously pumped by an argon laser. The ring structure with the saturable absorbent was called CPM for "colliding pulse mode-locking". The pulse duration was about 0.11 picoseconds.
It has subsequently been observed that synchronous pumping provides decisive advantages, using either a mode-locked YAG laser with frequency doubling (T. Norris, T. Sizer II, G. Mourou, J.O.S.A.B., 2, 613, 1985) or else using a mode-locked argon laser (M. C. Nuss, R. Leonhardt, W. Zinth, Optics Letters 10, 16, 1985). These advantages include the possibility of synchronous amplification and the possibility of pumping a standard tunable picosecond laser synchronously and in parallel.
Unfortunately, for a cavity of given characteristics, synchronously pumped lasers are much less stable than are continuously pumped lasers. In particular, a bistable one-way rotation phenomenon has been observed by A. M. Johnson and W. M. Simpson, Optics Letters 8, 554, 1983.
It is possible to avoid this behavior either by selecting a suitable sequence of pumping pulses (Nuss et al), or else by making use of a linear cavity together with an anti-resonant ring having the jet of saturable absorbent placed in the middle thereof (Norris et al).
However, even when improved in this way, femtosecond dye lasers with synchronous pumping remain extremely sensitive both in stability and in pulse duration to any slight mis-tuning in the length of the cavity.
For example, for 100 fs pulses, Nuss et al indicate that the length of the cavity must be adjusted to within 0.2 micrometers (i.e. to a relative value of better than 10.sup.-7).
At this level of accuracy, numerous factors have an effect:
instabilities in the pumping pulse rate, which ought to be avoidable, at least in theory;
thermal expansion and mechanical vibration of the components of the cavity; and
variations in the refractive index of air as a function of ambient humidity, or pressure, or temperature.
It is therefore practically impossible to maintain the desired accuracy for the optical length of the cavity, given that it is to be expected that this accuracy constraint will increase as the desired pulse duration is reduced.
The present invention helps solve this problem in a manner which would be impossible for passive temperature compensation.
A first aim of the invention is to obtain good stability both in the short term and in the long term (several hours) concerning the length or the duration of laser pulses.
Another aim of the invention is to improve the amplitude and frequency stability of a femtosecond laser.